Boat propeller shafts are typically subject to corrosion due to their constant immersion in water. The problem is most acute for propeller shafts in marine vessels due to the tendency of sea water to act as an electrolyte, causing galvanic corrosion.
A conventional inboard boat engine is mounted within the hull of a boat and linked to a propeller shaft that projects through a passageway in the hull and into the water below. The shaft passageway is typically fitted with a through-hull fitting mounted inside the hull adjacent the passageway and angled to accomodate passage of the rotating shaft therethrough. After entering the hull, the rotatable shaft passes through an inboard stuffing box, attached to the through-hull fitting by a flexible hose, to form a watertight seal around the shaft. The stuffing box typically includes a generally cylindrical housing, open on both ends, through which the rotating shaft passes. An annular fiber stuffing, or packing, material, which acts as a bearing for the rotating shaft and forms a watertight seal around the shaft, is placed onto the shaft and inserted into the stuffing box housing. An annular cover is then threaded, bolted, or otherwise secured to the stuffing box housing to compress the stuffing therein around the shaft.
The components of conventional stuffing boxes are dimensioned such that the rotating shaft comes into contact only with the substantially electrically insulative stuffing material. The end of the propeller shaft that projects into the water is further supported by an electrically insulative rubber bearing mounted within a strut fitting projecting from the underside of the hull. The inboard end of the propeller shaft is drivingly interconnected to the engine by an oil lubricated gear box that also provides a fair degree of electrical insulation. In larger vessels, the inboard portion of a propeller shaft may also pass through one or more bulkheads, disposed crosswise within the hull between the stuffing box and the engine. Each bulkhead is typically adapted with a bulkhead stuffing box configured similarly to that mounted on the hull to seal the passage of the shaft through the bulkhead. Conventional bulkhead stuffing boxes also tend to electrically insulate the propeller shaft.
As a result of the electrical insulation of the propeller shaft in conventionally configured vessels, the propeller shafts are subject to galvanic corrosion when immersed in fresh water and especially when immersed in salt water. Corrosion of the propeller shaft can result in weakening of the shaft or leakage of water past the shaft into the hull of a boat.
A conventional attempt to solve the problem of general corrosion in marine vessels involves cathodic protection of the metallic components of the vessel. Sacrificial anodes are mounted on the immersed portion of the hull or suspended by lines dropped in the water adjacent the hull exterior. The sacrificial anodes are formed from a metal that is less noble (more chemically reactive) than the metals desired to be protected on the vessel. Typical sacrificial anodes are formed of zinc, with magnesium or aluminum also sometimes being utilized. The anodes and the metal portions of the boat form a galvanic couple, electrically linked by seawater. The more noble metal on the boat acts as a cathode that is protected by corrosion of the less noble sacrificial anodes. Cathodic protection serves to generally protect most metallic components, including the propeller shaft. However, because the propeller shaft is electrically isolated, anodes must be mounted underwater directly on the shaft. A diver must be employed to install the anodes, or ultimately the vessel must be pulled from the water, both resulting in substantial expense.
A method for directly protecting the propeller shaft from corrosion is disclosed by U.S. Pat. No. 3,201,739, which teaches the electrical grounding of a rotating propeller shaft. The electrical grounding assembly discloses a generally semicircular conductive band that is positioned adjacent and surrounding one side of the propeller shaft. Two arcuate-shaped electrical brush contacts are mounted to the interior of the semicircular band and contact the shaft. Each end of the semicircular band is secured to a length of chain, with the chains joining at a point radially distant from the shaft and diametrically opposed from the band. The free end of the joined chains are connected to an anchor point. A spring mounted on the joined portion of the chain biases the brush contacts against the rotating shaft. An electrical lead is connected to the semicircular band to place the shaft in electrical contact with the grounding system of the boat. This mechanism is very cumbersome, involving several large parts that must be properly connected and which take up valuable room within the boat. In addition, the electrical brush contacts, while radially spaced around the circumference of the shaft, are longitudinally aligned along the axis of the shaft. The two contacts tend to wear a groove within the rotating shaft over the course of time, potentially shortening, the useful life of the shaft.